As I understand it, all DVC Members have access to Top of The World Lounge. However they need proof of their DVC membership. Also, they cannot park in the parking lot of the contemporary or Baylake tower to go to the top of the world lounge. If the top of the world lounge is crowded or on certain very busy days, it might possibly be restricted just to people who are guests at Baylake tower.
I believe that top of the world Lounge is considered to be owned by the DVC members. Therefore it is available to all DVC members, except for when it must be restricted because of excessive demand and usage. The day may come when it will be restricted only to people at Bay Lake Tower, but if this is the case, then it would probably only occur if there were too many people trying to use it. So far, though there are times when it is crowded and they do not let more people go up, on most days it is not in excessive demand.